keyListener未检测到按键

Question

我想围绕一个JFrame盒子移动一个圆圈图形，并决定添加一个KeyListener但我似乎无法让它工作。

package keyBoardInput;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Window;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.Rectangle;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class window extends JFrame implements KeyListener {

    Rectangle rect;

    //KeyListener keyListener;

    public void init() {

        this.addKeyListener(this);

        setFocusTraversalKeysEnabled(true);
        requestFocus(true);

        rect = new Rectangle(0,0,100,100);
    }

    public window() {

        super("Title bar");
        setSize(800,600);
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);

        setFocusTraversalKeysEnabled(true);
        requestFocus(true);
        setFocusable(true);
        //addKeyListener(keyListener);
    }

    public void paint(Graphics g){

        Graphics2D g2 = (Graphics2D)g;

        g2.setColor(Color.cyan);
        g2.fillRect(0,0,800,600);
        g2.setColor(Color.orange);
        g2.fillOval(0,0,100,100);
    }

    public void keyPressed(KeyEvent e) {

        System.out.println("test");
        int key = e.getKeyCode();

        if (key == KeyEvent.VK_W) {
            rect.setLocation(rect.x + 0, rect.y + 10);
        }
        if (key == KeyEvent.VK_S) {
            rect.setLocation(rect.x - 0, rect.y - 10);
        }
        if (key == KeyEvent.VK_A) {
            rect.setLocation(rect.x - 10, rect.y + 0);
        }
        if (key == KeyEvent.VK_D) {
        rect.setLocation(rect.x + 10, rect.y + 0);
        }

        repaint();
    }

    public void keyTyped(KeyEvent e) {}
    public void keyReleased(KeyEvent e) {}

}